Fuel pin spacer structure

ABSTRACT

A spacer structure is described for maintaining a spaced relation between a plurality of generally parallel fuel pins in a nuclear reactor. The spacer structure is comprised of a plurality of spacer elements constructed from flat plate stock and bent in a way so that when the separate pieces are assembled together, they support each fuel pin at a plurality of points and interlock and each other to form a grid-like assembly.

United States Patent 1 Bass [ 1 March 6, 1973 FUEL PIN SPACER STRUCTURE[75] Inventor: John C. Bass, La Jolla, Calif.

[73] Assignee: Gulf General Atomic Incorporated,

San Diego, Calif.

[22] Filed: Sept. 26, 1969 [21] Appl. No.: 861,336

[52] U.S. Cl ..l76/78, 176/76 [51] Int. Cl. ..G2lc 3/34 [58] Field ofSearch ..176/76, 78, 87, 73

[56] References Cited UNITED STATES PATENTS 3,137,638 6/1964 Kumpf et al..176/78 3,228,854 1/1966 Bekkering et a1. 176/78 3,255,091 6/1966Frisch ..l76/78 3,314,860 4/1967 Wilman ..176/78 3,398,053 8/1968 Huberet a1 1 ..176/78 3,423,287 1/1969 Anthony et 81.... ..176/78 3,431,1713/1969 Glandin ..176/78 Primary Examiner-Carl D. Quarforth AssistantExaminer-Gary G. Solyst Attorney-Fitch, Even, Tabin & Luedeka [57]ABSTRACT A spacer structure is described for maintaining a spacedrelation between a plurality of generally parallel fuel pins in anuclear reactor. The spacer structure is comprised of a plurality ofspacer elements constructed from flat plate stock and bent in a way sothat when the separate pieces are assembled together, they support eachfuel pin at a plurality of points and interlock and each other to form agrid-like assembly.

6 Claims, 9 Drawing Figures PATENTEL 11 73 SHEET 10F 2 FIG. 2.

FIG

INVENTOR.

JOHN C. BASS Adam F FUEL PIN SPACER STRUCTURE This invention relatesgenerally to nuclear reactors and, more particularly, to a spacerstructure for maintaining a spaced relation between a plurality ofgenerally parallel fuel pins in a nuclear reactor.

In nuclear reactors of the type which employ elongated fuel rods or pinsin the reactive core, provision is typically made for maintaining therelative location and alignment of the fuel pins. Typically, suchprovision is madeby means of a spacer structure comprised of elementsintegral with or separate from the fuel pins and which form a grid-likestructure which engages the outer surfaces of the fuel pins to maintainthe desired separation. Such a spacer structure may extend transverselyof the entire reactive core, or may extend transversely of a modularcore fuel element comprising an outer housing containing a plurality offuel pins.

In the proper design of a spacer structure for maintaining a spacedrelation between fuel pins in a nuclear reactor, a problem arises inconnection with so-called hot-spots on the cladding of the fuel pins. Inorder to remove heat from the fuel pins, a fluid coolant is typicallycirculated along the outer surface of the fuel pins. In the region wherea fuel pin is contacted by a spacer structure, however, contact betweenthe outer surface of the fuel pin and the circulating coolant may beprevented, not only in the region of contact, but in a larger regionadjacent thereto due to perturbation of the coolant flow by the presenceof the spacer structure. Overheating of a fuel pin in the region of ahot spot may cause serious trouble.

Another significant problem, in the design of spacer structures formaintaining a spaced relation between fuel pins in a nuclear reactor isthat of the coolant pressure drop resulting from the presence of aspacer structure. This problem may become particularly critical inconnection with reactors wherein the fluid coolant is a gas, since asubstantial pressure drop can have a significant effect on the heatremoved. Ideally, a spacer structure should present minimum resistanceto coolant flow. This factor is offset, however, by the need foradequate strength in the spacer structure to maintain the desired spacedrelationships. Various aerodynamic designs are possible in connectionwith spacer structures, however, this adds considerable expense to theirmanufacture.

As a result of the numerous variable factors which enter into the designof a spacer structure, the optimization of spacer design involves aconsideration and compromise of the many factors, the relativeimportance of which may depend upon the particular core design, type ofcoolant, etc. Some of the more important considerations in the design ofa spacer structure, in addition to the hot spot and pressure dropproblems discussed above, include: various considerations of mechanicaltolerances; the restraint which the structure provides against bowingtendencies of fuel pins as a result of differential thermal expansion;the desirability of restraining flow induced vibrations with associatedfretting and wear problems; localized stresses between the spacerstructure and the fuel pins; stresses within the spacer structureitself, and; the mass of additional metal introduced into the core bythe presence of the spacer structure.

It is an object of the present invention to provide an improved spacerstructure for maintaining a spaced relation between a plurality of fuelpins in a nuclear reactor.

Another object of the invention is to provide a spacer structure whichpresents minimal frontal area to coolant flow to thereby minimizepressure drop.

Another object of the invention is to provide a spacer structure whereinadequate mechanical support for the fuel pins is provided with minimumcontact area between the spacer structure and the fuel pins.

It is another object of the invention to provide an improved spacerstructure which is of low cost and which is easily manufactured.

Other objects of the invention will become apparent to those skilled inthe art from the following discussion, taken in connection with theaccompanying drawings wherein:

FIG. 1 is a perspective view of a modular fuel element (and a portion ofits supporting structure) with a portion of the fuel element housingbroken away to illustrate fuel pins and a portion of a spacer structureconstructed in accordance with the invention;

FIG. 2 is an enlarged plan cross sectional view taken along the line 2-2of FIG. 1;

FIG. 3 is an enlarged plan cross sectional view of a portion of the fuelelement as shown in FIG. 2;

FIG. 4 is an elevational view illustrating one spacer element employedin the spacer structure of the invention;

FIG. 5 is a perspective view of a spacer element used in an alternativeembodiment of the invention;

FIG. 6 is a plan sectional view illustrating an alternative embodimentof the invention employing spacer elements of the type shown in FIG. 5;

FIG. 7 is a perspective view of a modification of the spacer element ofFIG. 5;

FIG. 8 is a perspective view of a spacer element used in a furtheralternative embodiment of the invention; and

FIG. 9 is a plan sectional view of a further alternative embodiment ofthe invention utilizing spacer elements of the type illustrated in FIG.8.

Very generally, the spacer structure 1 1 of the invention includes aplurality of spacer elements 12, each being formed with at least twosections 13 having convex surfaces 15 thereon. Each convex surfaceprotrudes in a direction of a respective adjacent fuel pin 14 forengaging same. The sections of the spacer elements are of aconfiguration to interlock with the corresponding sections ofimmediately adjacent spacer elements and are so interlocked in agrid-like assembly. The spacer elements are formed with substantiallygreater dimension in a direction generally parallel with the axes of thefuel pins than in the direction transverse thereto to present, for agiven cross sectional area of the spacer elements, minimal resistance tocoolant flow along the fuel pins.

Referring now particularly to FIG. 1, a modular fuel element 16 isshown. The spacer structure 1 l of the invention which will be describedin connection with FIGS. 1 to 3 is for use in a modular fuel element ofthe illustrated type. A plurality of such fuel elements may bepositioned adjacent each other to form the reactive core for a nuclearreactor. It is to be understood, however, that the spacer structure ofthe invention is also applicable in situations other than thatillustrated, such as, for example, in situations wherein a core iscomprised of a plurality of mutually parallel fuel rods in a non-modulararrangement.

The modular fuel element 16 includes a box-like housing 17 which isshaped to enclose a hexagonal cross section. The upper end of thehousing 17 is provided with a section 18 of reduced size forming ashoulder 19 surrounding the periphery of the housing 17. The reducedsize section 18 mates in a correspondingly shaped recess in a supportingstructure (not shown) such that the fuel element, along with the othersimilar fuel elements, depends from the supporting structure and is heldin proper relative alignment. The fuel element 16 is secured in thesupporting structure, not shown, by suitable securing means 21 supportedat the top end of the fuel element housing 17 by diagonally extendingwebs 22. The support means 21 may, for example, be designed to provide abayonet type attachment to the supporting structure so that the modularfuel element may be readily released and lowered from the supportingstructure for replacement.

Inside the housing 17 of the modular fuel element 16, provision is made(not shown) for suspending a plurality of the fuel pins 14 from theirupper ends to depend downwardly and extend along the elongated housing17. The fuel pins 14 may be of any suitable construction, such as apelletized fissionable material contained within a suitable metalliccladding. The illustrated fuel pins 14 are cylindrical and are arrangedas shown in FIG. 2. It may be seen that, within the hexagonal housing17, the fuel pins are arranged in mutually parallel rows parallel withtwo of the opposite sides of the hexagon. The rows of fuel pins in thedirection perpendicular to such mutually parallel rows are also mutuallyparallel, but the fuel pins are spaced so that adjacent rows are offsetby one-half the diameter of a fuel pin; a generally hexagonal pitcharrangement.

In order to maintain a desired spaced relation between the fuel pins 14,a spacer structure 11 constructed in accordance with the invention isprovided at at least one location spaced along the length of the fuelpins from their supported upper ends. The spacer structure 11 iscomprised of a plurality of spacer elements 12. Each spacer element isfabricated out of sheet stock in a relatively simple and straightforwardprocedure. Basically, each spacer element includes three planar strips26 which extend at 120 angles with respect to each other from a commonaxis line 27 (see FIG. 4). The axis line 27, when the spacer element isassembled into the spacer structure, extends parallel with the axis ofthe fuel pins.

The strips 26 on each spacer element are displaced along the line 27with respect to each other, enabling the spacer element to be formed outof a blank of flat plate or sheet stock. To enable forming, the blank isprovided with an elongated slot 25 between the uppermost and lowermoststrips 26, and a stress relief hole 28 at the inner terminus of theslot. The uppermost and lowermost strips are then bent with respect toeach other out of their common plane in the blank to the relationshipillustrated. The blank is provided with a pair of angular cut-outs 29 toproduce a taper in the middle one of the strips 26 so that its contactsection l3 is of the same height as the other two contact sections onthe other two strips.

The contact sections 13 are at the end of each of the three strips 26 ina spacer element 12. In the spacer elements of the embodiments of FIGS.1 to 3, the convex surfaces 15 which protrude in the direction ofrespective adjacent fuel pins for engaging same are provided by formingthe contact sections 13 to have a pair of oblique planar segments 33 and34 (FIG. 3). The segment 33 is formed by producing a right-angle bend inthe corresponding strip 26, and the segment 34 is formed by producing afurther bent section extending from the segment 33 and forming a angletherewith. The convex surface 15 is thus formed between the segments 33and 34 and is preferably given a slight radius to minimize internalstresses at that point. The oblique planar segments 33 and 34 with thecurved section or convex surface 15 therebetween thus constitute atransverse protrusion extending from the plane of the associated strip26. These protrusions extend toward the most adjacent of the fuel pins14 to engage same on its outer surface at the convex surface 15 on thespacer element. This is most clearly shown in FIG. 3.

It may be seen from FIG. 2 that the strips 26 and the segments 33 and 34all lie in planes which extend parallel with the axes of the fuel pins14. Moreover, the identical nature of the angular relationships enablethe contact sections 13 to interlock with the corresponding contactsections of immediately adjacent spacer elements. By interlock, it ismeant not necessarily to be mechanically secure, but merely to cooperatewith each other in forming a desired configuration, such as a triangleor circle. In the embodiment of FIGS. 1-4, the corresponding contactsections which are interlocked are those sections on strips 26 of commonlevel, and form triangles. Thus, three intersecting contact sections 13will all be from strips 26 of either upper, middle, or lower levels asviewed in FIG. 4. The material of which the spacer elements arecomprised is selected to provide sufficient strength at operatingtemperatures and under conditions of high radiation. Moreover, thematerial is selected to be slightly resilient in order that the contactsections 13 may deform slightly when contacting the adjacent fuel pinsand thus maintain a bias force against the outer surface of the fuelpins. In the spacer structure 11 illustrated in FIGS. 1 to 4, the spacerelements assembled into the grid-like assembly are secured to each otherat their points of contact by suitably brazing or welding them.Alternatively, the ends of the planar segments 34 may be extended sothey abut the inside of the corresponding segment 34 on an adjacentspacer element. Thus, clockwise rotation of the spacer element as viewedin FIG. 2 is prevented and brazing or welding may not be necessary ifthe resilient bias provided by contact with the fuel pins is sufficient.Such an expedient, however, increases the frontal area presented tocoolant flow, as will be more thoroughly discussed subsequently, andthus may be undesirable in some circumstances.

In order to effect the transition from the general pattern of the spacerstructure to the peripheral shape of the structure as determined by theshape of the fuel element housing 17, transition pieces are provided atthe edge of the fuel pin array so that the spacer structure may beattached to the fuel housing. Such transition pieces may be seen in FIG.3 and are of three types. The various types of transition pieces areexemplified by the transition pieces 41, 42 and 43. The transition piece41 includes a pair of flat portions 44 which follow the inner contour ofthe hexagonal wall 17 of the modular fuel element 16. Connecting the twoflat pieces 44 is a ridge section 46 which projects inwardly and engagesthe periphery of the adjacent fuel pin 14.

The transition piece 42 similarly includes a pair of flat plate sections47. The flat plate sections 47 are supportive of a central projection 48of triangular cross section. The projection 48 mates on one side with acontact section 13 of an adjacent spacer element 12.

" The corner of the triangular section 48 other than that which mateswith the contact section 13 engages the outer periphery of an adjacentfuel pin 14 for supporting same.

The transition element 43 includes a flat section 51 which fits againstthe interior surface of the fuel element housing 17 and from whichextends, at a right angle, a flat strip 52. The strip 52 terminates in acontact section 53 which is identical with the contact sections 13. Thecontact section 53 of the transition piece 43 interlocks with othercontact sections on spacer elements 12 adjacent thereto and engages theadjacent fuel pin 14.

The flat plates 44, 47 and 51 of the transition elements 41, 42 and 43,respectively, are secured to the inner surface of the fuel elementhousing 17 by suitable means such as by brazing or welding. Of courseother configurations of transition elements are possible, and thevarious transition elements 41, 42 and 43 may be made integral with eachother in various convenient combinations. Moreover, the transitionelements 41 and 42 may be replaced with suitably formed indentations inthe wall of the fuel element housing 17.

In the design of nuclear reactor cores, wherein the fluid coolant iscirculated past the fuel pins or fuel rods, it is desirable to avoidexcessive drop in fluid pressure. Such a drop in fluid pressure canresult in deleterious performance, particularly where the fluid coolantbeing circulated is a gas. In order to avoid excessive pressure drop, itis desirable that obstruction to the flow of the fluid coolant beminimized. The spacer structure of the invention is manufactured out offlat plate stock and thereby presents, for a given cross sectional areaof the spacer element, minimum frontal area to the coolant. Thisminimizes the restriction presented in such coolant flow, because thegreater dimension of the spacer element is in the direction generallyparallel with the axes of the fuel pins. Since the various elements ofthe spacer structure are fabricated from flat plate stock and bent toparticular angles, the fabrication cost is minimized.

Another significant advantage arising from the invention is theminimization of so-called hot spots on the fuel pin cladding. A largenumber of variables have an affect on the extent of hot spots and amongthese variables are:

l. Spacer-to-fuel-pin pin cladding contact heat conductance;

2. Local circumferential variations in the film heat transfercoefficient in the vicinity of the spacer-tofuel-pin contact;

Spacer material thickness and thermal conductivity;

4. The effects of axial conduction on heat in the spacer structure andthe fuel rod;

5. The linear heat output of the fuel rod;

6. Axial variations in film coefficient due to spacer induced flow fieldperturbation;

7. Spacer length.

With respect to the above variables, one of the most significantvariables is the net effective spacer-to-fuelpin cladding contact heatconductance. In the design of the invention, the actual contact area isminimal, exposing a maximum amount of pin surface to the coolant. Forcertain effects, the technique of surface roughening may be employed toincrease the heat con- -duction in the contact area and thereby furtherenhance cooling. The actual nature of the heat conduc tion under somecircumstances is highly complex, being a combination of both contacttransfer and convection through the spaces where the parts are not incontact clue to roughening.

A second highly significant parameter is the local circumferentialvariation in film coefficient in the immediate vicinity of thespacer-element contact due to the shrouding effect of the spacer itself.The shrouding effect of the spacer is that effect resulting from thedeflection of the flowing coolant away from the surface of the fuel pinin the region of spacer-element contact. The particular configuration ofthe spacer structure of the invention places a minimum of materialadjacent the contact region and thereby minimizes the amount of fluidcoolant deflection at such region.

The third variable listed above, that of spacer material thickness andthermal conductivity, depends upon the particular structuralrequirements. A material should be selected to be relatively thin tominimize coolant pressure drop and should have the maximum thermalconductivity within the structurally feasible materials.

The fourth factor, the effects of axial conduction, are minimal in gridtype spacers such as is the general nature of the structure of theinvention. The same is true of the fifth factor listed above.

In view of the fact that the spacer structure of the invention presentsminimum frontal area to the coolant flow and minimizes the amount ofmaterial closely adjacent the fuel pin surface, perturbation of thecoolant flow, both generally and near the region of contact, isminimized. As a result, axial variations in the film coefficient due toperturbations of the coolant flow field are minimized.

The seventh factor above, spacer length, is typically insignificant inspacer structures designed in accordance with the invention. This isprobably due to the inherent nature of the grid type spacer in whichaxial conduction effects are minimal.

To summarize the advantages of the invention, the size of the affectedzone on the surface of the fuel pin which is affected by local heattransfer variation, and which is generally proportional to the hot spotwhich could develop, is minimized for the spacer structure of theinvention. Another advantage of the invention is that of the flexibilityprovided, since the spacer struc ture of the invention is not restrictedto particular pin pitch-to-diameter diameter ratios. When using atriangular pin pitch, such as is common in the art, there is a limit onpitch-to-diameter ratio at which one cannot get a straight path betweenthe fuel pins. As a result of this limit, the pitch-to-diameter ratio ofthe spacer in a triangular pin pitch arrangement is typically limited tonot less than 1.155. In the spacer structure of the invention, thepitch-to-diameter ratio can be significantly less, and can even approach1, the limit, of course, being governed by the thickness of the metalfrom which the parts are fabricated.

A further advantage of the invention is, as previously stated, theminimal cost of manufacture. In addition, the particular spacerstructure of the invention readily accommodates fuel pin twisting due tothermal distortion or tortional vibration with no diminution in thefunctionality of the spacer structure. The flexible nature of 'thespacer also takes into account some tolerance variation and may aid indamping flow induced vibrations.

Referring now to FIGS. and 6, an alternative embodiment of the inventionis illustrated. The arrangement of FIG.,6 provides six points of contactfor each fuel pin rather than three as in the previously describedembodiment. Referring to FIG. 5, a single spacer element 56 isillustrated comprising a central flat strip section 57 and a pair ofcontact sections 58, one on each end of the strip 57. The contactsections 58 are identical with the contact sections 13 shown anddescribed in connection with the previous embodiment, and interlock withthe corresponding contact sections of immediately adjacent spacerelements in the same manner as the previous embodiment. The resultinginterlocking arrangement of separate spacer elements 56 may be seen inFIG. 6 in which the spacer elements are interlocked in a grid-likeassembly and in which the convex surfaces provided by the contactsections 58 engage the fuel pins 59 in the manner of the previousembodiment.

Separate transition elements, not shown, are provided as in the previousembodiment at the periphery of the spacer structure engaging the innersurface of the fuel element housing. The arrangement illustrated in FIG.6 has the advantage that the individual spacer elements 56 are easy tomake and that the spacer structure itself gives greater support due tothe greater number of contact regions. On the other hand, the greatersurface area presented to the coolant flow is a disadvantage since moreflow resistance results. Where pressure drop is not highly critical,such a structure may be of significant advantage.

As a modification of the design of the spacer element illustrated inFIG. 5, a spacer element is shown in FIG. 6 wherein only the top half ofa strip 57a is formed with contact sections 58a on its ends, and whereinthe bottom half 57b extends in a straight line with only a small offsetsection 58b. The sections 58b on adjacent spacer elements engage eachother. The individual spacer elements may then be secured to each otherby brazing at the engaged sections 58b of the lower parts 57b, while thecontact sections are left unbrazed for greater flexibility. The otherembodiments described herein may be modified in a similar manner.

Referring now to FIGS. 8 and 9, a further alternative embodiment of theinvention is illustrated. The embodiment of FIGS. 8 and 9 is similar tothat of FIGS. 5 and 6 in the provision of a strip-like spacer element 66having a pair of contact sections 67, one at each end. each end. Thecontact sections of the spacer element 66, however, are of substantiallysemi-cylindrical shape rather than the angular shape of the embodimentof FIGS. 5 and 6. Accordingly, a slightly greater amount of frontal areais presented to the flow of coolant and a slightly greater amount ofmaterial is in the region of contact between the spacer element and theadjacent fuel pin. In some instances however, such increases may bewithin the tolerable limits. To allow for variation in the radii of thecontact sections, it may be desirable to provide a bushing or ferrule 68(FIG. 9) in the region where the contact sections 67 interlock. Thisadds to the total amount of frontal area presented to the coolant flowbut may be tolerable in some circumstances.

It may therefore be seen that the invention provides an improved spacerstructure for maintaining a spaced relation between a plurality ofgenerally parallel fuel pins in a nuclear reactor. The structureprovidesvery little flow restriction and minimum coolant perturbationnear the fuel pin, which minimizes hot spots. Since it is fabricatedfrom flat sheet stock and bent to particular angles or radii, thefabrication cost is minimal. When utilized in a hexagonal array, thearray of fuel pins is not limited to particular pitch-to-diameterratios. The invention has particular advantage in connection with gascooled nuclear reactors, but is applicable to nuclear reactors of alltypes utilizing elongated fuel pins or fuel rods, with or withoutmodular construction.

Various modifications of the invention in addition to those shown anddescribed herein will become apparent to those skilled in the art fromthe foregoing description and accompanying drawings. Such modificationsare intended to fall within the scope of the ap pended claims.

What is claimed is:

l. A spacer structure for maintaining a spaced relation between aplurality of generally parallel fuel pins within a housing in a nuclearreactor, comprising, a plurality of spacer elements with the housinghaving a lateral cross section substantially larger than the size of asingle one of said spacer elements, said each of said spacer elementsbeing formed with at least two contact sections having convex surfacesthereon, each convex surface protruding toward a respective adjacentfuel pin for engaging same, at least part of said contact sections ofsaid spacer elements being formed in an interlockable configuration tointerlock with the corresponding contact sections of immediatelyadjacent spacer elements and being so interlocked in an interlockedassembly, each interlocked assembly having three extending planarstrips, wherein one of said contact sections is at the end of eachstrip, and wherein said spacer elements are arrangedin a grid structurewith each interlocked assembly being engaged at three of said contactsections by different fuel pins said spacer elements being formed withsubstantially greater dimension in the direction generally parallel withthe axes of the fuel pins than in the direction transverse thereto topresent, for a given cross sectional area of said spacer elements,minimal resistance to coolant flow along the fuel pins.

2. A spacer structure according to claim 1 wherein each spacer elementis of elongated shape, wherein one of said contact sections is at eachend thereof, and

5. A spacer structure according to claim 4 wherein each of said contactsections includes a substantially semi-cylindrical segment.

6. A spacer structure according to claim 1 wherein each spacer elementis fabricated out of plate metal with three extending strips of platemetal each of which is offset from the other two strips a distance inthe direction generally parallel with the axes of the fuel pins, saidspacer elements being arranged in said gridlike assembly with the legsof corresponding displacement at corresponding levels.

1. A spacer structure for maintaining a spaced relation between aplurality of generally parallel fuel pins within a housing in a nuclearreactor, comprising, a plurality of spacer elements with the housinghaving a lateral cross section substantially larger than the size of asingle one of said spacer elements, said each of said spacer elementsbeing formed with at least two contact sections having convex surfacesthereon, each convex surface protruding toward a respective adjacentfuel pin for engaging same, at least part of said contact sections ofsaid spacer elements being formed in an interlockable configuration tointerlock with the corresponding contact sections of immediatelyadjacent spacer elements and being so interlocked in an interlockedassembly, each interlocked assembly having three extending planarstrips, wherein one of said contact sections is at the end of eachstrip, and wherein said spacer elements are arranged in a grid structurewith each interlocked assembly being engaged at three of said contactsections by different fuel pins said spacer elements being formed withsubstantially greater dimension in the direction generally parallel withthe axes of the fuel pins than in the direction transverse thereto topresent, for a given cross sectional area of said spacer elements,minimal resistance to coolant flow along the fuel pins.
 1. A spacerstructure for maintaining a spaced relation between a plurality ofgenerally parallel fuel pins within a housing in a nuclear reactor,comprising, a plurality of spacer elements with the housing having alateral cross section substantially larger than the size of a single oneof said spacer elements, said each of said spacer elements being formedwith at least two contact sections having convex surfaces thereon, eachconvex surface protruding toward a respective adjacent fuel pin forengaging same, at least part of said contact sections of said spacerelements being formed in an interlockable configuration to interlockwith the corresponding contact sections of immediately adjacent spacerelements and being so interlocked in an interlocked assembly, eachinterlocked assembly having three extending planar strips, wherein oneof said contact sections is at the end of each strip, and wherein saidspacer elements are arranged in a grid structure with each interlockedassembly being engaged at three of said contact sections by differentfuel pins said spacer elements being formed with substantially greaterdimension in the direction generally parallel with the axes of the fuelpins than in the direction transverse thereto to present, for a givencross sectional area of said spacer elements, minimal resistance tocoolant flow along the fuel pins.
 2. A spacer structure according toclaim 1 wherein each spacer element is of eloNgated shape, wherein oneof said contact sections is at each end thereof, and wherein said spacerelements are arranged in said grid-like assembly having a plurality ofopenings for receiving fuel pins, said spacer elements being arrangedwith six of said convex surfaces projecting into each of said openings.3. A spacer structure according to claim 1 wherein each spacer elementis comprised of sheet metal, and wherein each of said contact sectionscomprises a transverse formed protrusion in the sheet.
 4. A spacerstructure according to claim 4 wherein each of said contact sectionsincludes a pair of oblique planar segments.
 5. A spacer structureaccording to claim 4 wherein each of said contact sections includes asubstantially semi-cylindrical segment.